Torino Impact Hazard Scale

A risk-assessment scale, with integer values from 0 to 10, used to describe an approach of a
particular asteroid to Earth in terms of the likelihood and
consequences of its collision with Earth. The scale was devised
by Richard P. Binzel and first presented at a conference in 19951. A
revised version was approved at an International Astronomical Union (IAU) workshop in Torino
(Turin), Italy in 1-4 June 1999 and formally adopted
by the IAU in an announcement at the UNISPACE
III conference in Vienna on 22 July 1999.

Category

Index

Description

Events Having No Likely Consequences

0

The likelihood of a collision is zero, or well below the chance that a random object of the same size will strike the Earth within the next few decades. This designation also applies to any small object that, in the event of a collision, is unlikely to reach the Earth's surface intact.

Events Meriting Careful Monitoring

1

The chance of collision is extremely unlikely, about the same as a random object of the same size striking the Earth within the next few decades.

Events Meriting Concern

2

A somewhat close, but not unusual encounter. Collision is very unlikely.

3

A close encounter, with 1% or greater chance of a collision capable of causing
localized destruction.

4

A close encounter, with 1% or greater chance of a collision
capable of causing regional devastation.

Threatening Events

5

A close encounter, with a significant threat of a collision capable of causing
regional devastation.

6

A close encounter, with a significant threat of a collision capable of causing a global catastrophe.

7

A close encounter, with an extremely significant threat of a collision capable of causing a global catastrophe.

Certain Collisions

8

A collision capable of causing localized destruction. Such events occur somewhere on Earth between once per 50 years and once per 1000 years.

9

A collision capable of causing regional devastation. Such events occur between once per 1000 years and once per
100,000 years.

10

A collision capable of causing a global climatic catastrophe. Such events occur once per 100,000 years, or less often.

The index number is derived from two factors: the kinetic energy of the
asteroid, and the probability of a collision with Earth, according to the
following chart.

The left-hand scale is in megatons of TNT
(but they define the megaton as 4.3 × 1015 joules). The secondary
left-hand scale shows the diameter of the object, assuming typical encounter
velocities.

According to Binzel, speaking in 1999, no asteroid identified to
that date had a value greater than 1. It often happens that an approach's
initial rating is downgraded as additional observations make it possible to
define the asteroid's orbit more precisely. For information on asteroids
currently of interest, see: